The endogenous viruses of the guinea pig include the guinea pig virus (GPV), the intracisternal A particle, and at least two herpes viruses. We are currently utilizing a sensitive radioimmunoassay to detect the presence of GPV p27 in normal and malignant guinea pig cells. Certain permanent cell lines, and leukemia tumor cells, contain significant levels of p27, whereas normal guinea pig cells at low passage levels contain little or no p27 unless treated with BrdU. Guinea pig bone marrow cultures are being used as a model system to study the ability of viruses and chemicals to act as carcinogens or co-carcinogens.